An analytic formulation of transport in isolated, perfused renal tubules is proposed. The principal route of transepithelial water flow is postulated to be via a transcellular pathway across the luminal cell surface and cytoplasm, then across lateral cell walls and into intercellular channels. Using quantitative morphometric techniques, we have found that the series membranes in the transport pathway are identical in area but that the areas and the shape of the channels differed in segments having different function. On the basis of this information we have postulated a dependent relationship between cell shape and tubule water reabsorption. Furthermore, that relationship is found to be consistent with two principal driving forces for transepithelial water flow. One force results from an effectively symmetric distribution of active transport processes in the channel walls. Another force results from the retrograde flow of peritubular serum proteins into the channels with consequent oncotic forces across lateral cell walls.